The use of suspended particles to enhance indirect heat transfer between fluids is taught, for example, in U.S. Pat. No. 2,690,591 to Peskin, issued Sept. 29, 1954. Peskin describes the use of suspended particles to increase heat transfer efficiency in closed cycle thermodynamic systems.
U.S. Pat. No. 3,991,816 to Klaren, issued Nov. 16, 1976, teaches a method and apparatus for exchanging heat between a moving fluid and a moving secondary fluid comprising passing a plurality of streams of secondary fluid upward through the moving primary fluid. Solids are suspended in the secondary fluid.
U.S. Pat. No. 4,176,710 to Gansauge et al. discloses a fluidized-bed reactor containing at least one vertical transfer tube for conveying a heat transfer fluid through the fluidized bed.
U.S. Pat. No. 4,403,650 to Klaren, issued Sept. 13, 1983, discloses a vertical shell and tube heat exchanger in which solid particles flow from a lower chamber to an upper chamber through the heat exchanger tubes and are returned to the lower chamber via a valved conduit.
The article "Fluidized-bed heat exchanger avoids fouling problems" Vol. 94, CHEMICAL ENGINEERING 43 (Feb. 15, 1988) discloses the use of finely divided particles to minimize heat exchanger tube fouling.
Transferring heat to a fluidized-bed reaction zone poses a particularly perplexing engineering problem. The most economically advantageous hydrocarbon conversion reactions carried out in such an environment include dehydrogenation and aromatization, both generally endothermic and both requiring high temperatures for acceptable yields. Aromatization of C.sub.3 -C.sub.8 paraffins over a zeolite catalyst having the structure of ZSM-5, for example, requires a heat input of 350-1500 BTU per pound of feed at a reaction temperature of about 450.degree. C. to 700.degree. C. (824.degree. F. to 1292.degree. F.). The limitations of previously known heat transfer techniques have impeded the commercial development of such high temperature fluidized-bed conversion processes. Methods known in the art to transfer heat to the fluidized-bed reaction zone included preheating the catalyst or positioning a heat exchanger in the fluidized catalyst bed. Preheating the catalyst separately to around 700.degree. C. (1292.degree. F.) undesirably accelerated catalyst deactivation. On the other hand, the heat transfer coefficient between circulating fluids and the inner walls of heat exchanger tubes was found to be too low for economic operation. Thus it can be seen that it would be highly desirable to increase the efficiency of heat transfer to such fluid-bed reaction zones.